1. Field of the Invention
The invention generally relates to a projection lens, and more particularly, to a fixed-focus lens.
2. Description of Related Art
Usually, if a projector is used to project an image to a larger screen, a longer projection distance is required. Contrarily, if projector is used to project an image with a shorter projection distance, a specific wide-angle lens is required to shorten the distance between the projection screen and the projector.
In the design of a wide-angle lens, aberration is a difficult issue a designer needs to be confronted with. In order to reduce the aberration, a designer often disposes a reflection mirror to shorten the focal length and uses more lenses, for example, as shown by U.S. Pat. No. 7,009,765 and No. 7,175,287. In U.S. Pat. No. 7,009,765, the designer uses 16 lenses and a reflection mirror, while in U.S. Pat. No. 7,175,287, the designer uses 18 lenses, two reflection mirrors and a component with a bent optical path.
On the other hand, a designer can adopt a plurality of reflection mirrors to shorten the projection distance, as shown by U.S. Pat. No. 5,477,394, No. 6,994,442, No. 6,989,936, No. 6,690,517 and No. 6,984,044. However, the increasing reflection mirrors would increase the production cost and assembly difficulty.
In more details, U.S. Pat. No. 5,477,394 discloses a small lens structure, wherein the distance between a light valve to the imaging plane is shorter. The above-mentioned structure is able to converge the light reflected by the light valve at the primary focus of an ellipse reflection mirror, and then, the light is transmitted to the secondary focus of a parabola reflection mirror and a projection lens. In addition, U.S. Pat. No. 5,495,306 also provides a similar lens structure. In U.S. Pat. No. 6,994,442, the lens structure includes a plurality of reflection mirrors with aspheric curve surfaces.
U.S. Pat. No. 6,690,517 discloses an optical system, which includes a reflection lens group, a reflection mirror perpendicular to the optical axis and a lens set. The reflection lens group includes an aperture and the lens set includes at least a refractive surface with a negative refractive power. In U.S. Pat. No. 6,989,936, the designer provides a variable optical system, which includes a first optical component and a second optical component. The first optical component has a reflective surface and the second optical component, in comparison with the first optical component, contributes more to reduce the Seidel aberration.
Besides, a fixed-focus lens is disclosed, which includes a curved reflection mirror, a first lens group and a second lens group, wherein the first lens group includes an aspheric lens. Taiwan Patent Publication No. 200914873 discloses an optical projection system, which includes an aspheric lens, wherein the summation of the distortion coefficients of the Seidel aberration of the two surfaces in the above-mentioned aspheric lens is positive and greater than 0.5.
U.S. Pat. No. 4,427,274 discloses an optical system which includes a concave reflection mirror and a wide-angle lens. The concave reflection mirror is suitable for an aperture and the wide-angle lens is disposed along the optical axis of reflection mirror. U.S. Pat. No. 6,631,994 discloses an optical system able to correct the pillow-shaped distortion. In addition, U.S. Pat. No. 6,984,044 and No. 7,075,727 respectively disclose a projection system able to reduce the distortion.
Besides, U.S. Pat. No. 7,370,977, No. 7,586,687, No. 6,513,935, US Patent Application No. 20070184368 and No. 20090231555 also disclose several technologies related to the projection lens.
It can be seen from the depiction above that how to design a lens which is able to take a balance consideration between the lower production cost and the better imaging quality has become a significant developing point for the related developers.